Many wireless communication systems use base stations to provide geographical service areas where wireless communication user equipment (UE) devices communicate with the base station providing the particular geographical service area in which the wireless communication UE devices are located. The base stations are connected within a network allowing communication links to be made between the wireless communication devices and other devices. In some circumstances, the communication links are between UE devices that are close to each other. In these situations, it may be preferred to have a direct communication link between the two UE devices rather than communicating through a base station. Such direct communication between devices is often referred to as device-to-device (D2D) communication or peer-to-peer (P2P) communication.
The frequency spectrum used by a particular communication may be licensed or unlicensed. Licensed frequency spectrum (frequency band) is licensed to a system operator by a government agency, such as the Federal Communications Commission (FCC). An example of such licensed spectrum includes frequency bands used for cellular communication. An unlicensed frequency band is any portion of frequency spectrum that does not require a license from the government agency to communicate with the unlicensed frequency band. Equipment operating within the unlicensed band, however, typically must adhere to regulations and/or communication standards. An example of unlicensed frequency spectrum includes frequency bands used for Institute of Electrical and Electronics Engineers (IEEE) 802.11 communication.
Communication systems operating in a licensed frequency band typically utilize timing schemes or timing structures for managing communications between devices. Time can be defined in any number of units and subunits where the communication equipment applies the timing to transmission and reception. For example, some systems divide time into at least frames, subframes, timeslots, and symbol times. The physical channel structure used by a system may also involve dividing and organizing the available frequency bandwidth. As a result, a defined physical channel structure typically includes dividing the frequency and time resources into units and subunits to allow efficient management and use of time-frequency communication resources.